This specification relates to interferometric optical techniques to characterize profiles of various test surfaces, e.g., toric test surfaces.
Interferometric optical techniques are widely used to measure optical thickness, surface shape, and other geometric and refractive index properties of precision optical components such as glass substrates used in lithographic photomasks, optical lenses, air bearing surfaces of magnetic head sliders, etc.
For example, one can use an interferometer to combine a test beam reflected from a test surface with a reference beam reflected from a reference surface to form an optical interference pattern (also referred to as a fringe pattern). Spatial variations (e.g., dark and bright fringes) in the intensity profile of the optical interference pattern correspond to phase differences between the combined wave fronts of the test and reference beams caused by, for example, variations in the profile of the test surface relative to the reference surface.
Phase-shifting interferometry (PSI) can be used to accurately determine phase differences and a corresponding profile of a measurement surface based on a series of optical interference patterns recorded for each of multiple phase-shifts between the wave fronts of the test and reference beams. For each spatial location of the pattern, the series of optical interference patterns define a series of intensity values, also referred to as interference signal, which depend on the phase difference between the combined wave fronts of the test and reference beams for that spatial location. Using data processing techniques known in the art, the phase difference can be extracted from the interference signal for each spatial location. These phase differences can be used to determine information about the test surface including, for example, a profile of the measurement surface relative the reference surface. Such data processing techniques are referred to as phase evaluation algorithms.
Varying phase-shifts in PSI can be produced by various ways that change the optical path length from the test surface to the interferometer relative to the optical path length from the reference surface to the interferometer. For example, the reference surface can be moved relative to the test surface or a modulator may be placed in one of the beam paths. Alternatively, phase-shifts can be introduced for a constant, non-zero optical path difference by changing the wavelength of the test and reference beams.